This invention relates generally to techniques for using secondary surveillance radar to identify and determine the location of a target such as an aircraft. More particularly, this invention relates to techniques for distinguishing real targets from reflected targets and for generating a map of all radar reflector objects in the secondary surveillance radar region.
An air traffic control radar system typically includes a primary surveillance radar system and a secondary surveillance radar (SSR) system. Both systems can determine the range and direction of an aircraft from the radar installation. A secondary surveillance radar system, however, can also identify each aircraft using a specific code reported by that aircraft.
The primary and secondary radar systems can be either collected to operate together, or they may operate autonomously. The primary surveillance radar system uses a primary antenna mounted on a tower to transmit electromagnetic waves. The primary antenna rotates continuously to scan a selected surveillance region. These electromagnetic waves are then reflected or xe2x80x9cbounced backxe2x80x9d from an object (such as an aircraft). This reflected signal is then displayed as a xe2x80x9ctargetxe2x80x9d on the air traffic controller""s radarscope. The primary surveillance radar system measures the time required for a radar echo from the aircraft to return to the primary radar antenna. The primary surveillance radar system also measures the direction and height of the echo from the aircraft to the primary radar antenna. Secondary surveillance radar was originated in WWII to add the capability of distinguishing friendly aircraft from enemy aircraft by assigning a unique identifier code to the friendly aircraft. The system was initially intended to distinguish between enemy and friend but has evolved such that the term xe2x80x9cidentify friend or foexe2x80x9d (IFF) commonly refers to all modes of SSR operation, including civil and foreign aircraft use.
The secondary surveillance radar system, also known as beacon radar, uses a secondary radar antenna. In most installation when the two radar systems are co-located, this secondary antenna is attached to the primary radar antenna. However, the SSR system can operate in an autonomous installation where the SSR system is used for the radar surveillance task. The SSR antenna is used to transmit the interrogation calls and to receive the aircraft data. Military and commercial aircraft have transponders that automatically respond to a signal from the secondary surveillance radar interrogation with an identification code and altitude. The code is a predetermined message in response to a predefined interrogation signal. Before an aircraft begins a flight, it receives a transponder code from an air traffic controller. Normally only one code will be assigned for the entire flight. These codes are sometimes called mode codes. The range to the target is calculated from the time delay between the interrogation and the response time. Thus the SSR system provides for friendly aircraft, all the data that primary radar can provide, and more.
There are five major modes of operation and one sub-mode currently in use in the United States. Mode 1 is a nonsecure low cost method used by ships to track aircraft and other ships. Mode 2 is used by aircraft to make carrier-controlled approaches to ships during inclement weather. Mode 3 is the standard system used by military and commercial aircraft to relay their positions to ground controllers throughout the world for air traffic control (ATC). Mode 4 is used for secure encrypted IFF. Mode xe2x80x9cCxe2x80x9d is the altitude encoder. Mode S is a new IFF procedure for both military and civilian air traffic control that includes transmission of other data in addition to the mode code. The non-secure codes are manually set by the pilot but assigned by the air traffic controller.
A secondary surveillance radar system includes three main components: an interrogator, a transponder and a radarscope. In an air traffic control radar system, the interrogator, a ground based radar beacon transmitter-receiver, scans in synchronism with the primary radar and transmits discrete radio signals that repetitiously request all transponders on a selected mode to reply. The replies received are then mixed with the primary returns, and both are displayed on the same radarscope.
The transponder on an aircraft has an omni-directional antenna so that it can receive and reply to a radar signal from any direction. The transponder receives the signals from the interrogator and selectively replies with a specific pulse group (code) only to those interrogations being received on the mode to which the transponder is set. These replies are independent of primary radar returns, which are received from the target xe2x80x9cskinxe2x80x9d return. The replies processed by the SSR interrogator for display are sometimes called xe2x80x9cplots.xe2x80x9d The radarscope used by the controller displays returns from both the primary radar system and the secondary radar system. These returns are what the controller refers to in the control and separation of air traffic.
It is known that the secondary surveillance radar (SSR) suffers from a target reflection problem where a single target may be reported in several directions during one antenna scan. Only one position is the correct one for the target, and the others are xe2x80x9cphantomxe2x80x9d images that confuse the radar operator. Ground objects that act as electromagnetic xe2x80x9cmirrorsxe2x80x9d reflect the electromagnetic wave to the target and back to the SSR system generate these reflections. These reflector objects can be comprised of any electrically conductive material located in the proximity of the radar site (buildings, hangars, metallic fences, etc.). The problem is much more significant in an SSR system than in primary radar. The SSR transponder generates a high signal level that is not sufficiently attenuated by the interrogator one-way receiving antenna. The primary radar skin return is much weaker, attenuated faster as a function of radar range and is attenuated by the two-way antenna beam (versus one-way antenna beam of the SSR system). In some typical test conducted the number of SSR false reports can be as high as 30% of the total target reports.
The false target is generated when the SSR directional radar antenna is pointed at a reflector object rather than to the real target. The interrogator signal is reflected from the reflector object that acts as a mirror, toward the real target. The transponder in the target emits signals in all directions including the direction of the ground reflector. This signal is now reflected back from the same reflector back toward the SSR system resulting in a false target reported at the direction of the ground reflector. As a result, a target may appear on the radar screen in all azimuths where ground reflectors exist. To make the situation more complicated, unlike in primary radar systems where the ground reflectors are mapped by the radar surveillance, they are not visible by the SSR system, which responds only to active target code reports.
Although current SSR systems contain processes to reduce the number of false target reflections, the final results are not satisfactory. Receiver gain reduction at shorter range, Gain Time Control (GTC), may reduce the number of false targets at short ranges (at the expense of height coverage at those ranges). There is a false target rejection algorithm that requires complete mapping of all reflectors in the surveillance area including their electromagnetic properties. This is a very time-consuming task, with limited accuracy and will not provide a solution for the case where reflectors are dynamically changed (car on the road, new structures built or reflection conditions change due to changes in electromagnetic properties). An automatic technique that rejects all false targets and required no prior knowledge of the reflectors in the surveillance area is presented in this invention.
One aspect of the present invention identifies and rejects all secondary surveillance radar (SSR) plots reported from a reflected target without the need for prior knowledge of the locations of reflector objects in the surveillance region. Another aspect of the invention identifies and maps all reflector objects in the surveillance region using only targets that are present in the surveillance region. The present invention provides reliable identification of real targets, while eliminating the need for costly and lengthy flight tests and a site survey currently required for SSR radar installation.
A method according to the invention for identifying false target signals on a radar display caused by reflection of radar signals from a reflector object in a surveillance region using a radar system that includes a radar interrogator comprised of a radar transmitter/receiver arranged to display plots of radar signals that indicate positions of targets in the surveillance region, comprises the steps of obtaining a first plot of radar data for target position at a first time and obtaining a second plot of radar data for target position at a second time. The first and second plots of radar data are compared to determine whether they represent multiple reports of a single target or whether they represent different targets. The method also includes the steps of determining a first range from the radar interrogator for the first plot of radar data if the first and second plots of radar data represent multiple reports of a single target and determining a second range from the radar interrogator for the second plot of radar data if the first and second plots of radar data represent multiple reports of a single target. The first and second ranges are compared to determine which has the larger magnitude with the larger magnitude; and the plot of radar data having the larger range is identified as being a false target signal.
The step of comparing the first and second plots of radar data may comprise the steps of subtracting the first time from the second time to obtain a time difference for the first and second plots of radar data, calculating a target velocity for the second plot of radar data; and using the target velocity of the second plot of radar data and the time difference to propagate the range for the second plot of radar data to the same time as the first plot of radar data.
The invention may further include the steps of subtracting the first range from the second range to determine a range difference, comparing the range difference to a threshold, and identifying the second plot of radar data as being a false target signal if the range difference exceeds the threshold.
The invention may also further include the step of processing signals input to the radar display to block false target signals that have been identified.
The invention may include calculating a range of the reflector object from the radar interrogator. The method may also include calculating a facing angle of the reflector object relative to the interrogator as a function of the azimuth, range, and altitude measurements. The invention may include storing the range of the reflector object from the interrogator and the facing angle of the reflector object in a database.